This invention relates generally to terrain height measurement, and more specifically, to methods and systems for measuring a terrain height which take into account seasonal and other variations in the terrain.
Aircraft precision landing systems based on Global Navigation Systems (GNSs) such as the global positioning system (GPS) and Spaced Based Augmentation Systems (SBASs) such as the Wide Area Augmentation System (WAAS), which is a form of differential GPS, generally face a greater challenge for accuracy and integrity of vertical position (i.e., altitude above sea level) measurements than for horizontal position (i.e., latitude and longitude) measurements. As such, barometric altimeters and GNS/SBAS receivers are not sufficiently accurate for some precision landing operations.
Landing systems have been proposed to improve altitude accuracy by augmenting GPS/WAAS systems with a radar altimeter that is combined with a digital terrain database. The radar altimeter/digital terrain database combination calculates altitude, for example, relative to sea level, by adding a radar altimeter measured altitude, that is relative to terrain height, to a terrain height that is relative to sea level, as tabulated in the digital terrain database. However, the accuracy of altitude derived from radar altimeter/digital terrain database can vary from one geographical region to another.
One reason for such variances is due to the variations in surface conditions over time. For example, a forest can cause the radar altimeter to measure an altitude with respect to the height of the tree tops in summer when trees are leafed out. The same radar altimeter may measure the altitude with respect to the ground when leaves are down. This seasonal effect can cause significant variations, for example, in excess of forty feet, in the radar altimeter/digital terrain database calculated altitude.
Another reason for such variances is that the terrain may be surveyed more accurately and/or with higher resolution over some geographical areas than over other areas. For example, the terrain may be surveyed very accurately and with high resolution near a major airport, but have lower accuracy and/or resolution in remote areas, causing spatial variations in the accuracy of the digital terrain database.
Further, terrain is generally not flat within the region represented by each point in the digital terrain database. For example, the region may include hills and structures. Some digital terrain databases provide height above sea level for the highest point within the region represented by that database entry. A single terrain height measurement may not accurately reflect the terrain height over the entire region.
Horizontal errors from the aircraft's navigation system may also contribute to altitude errors derived from radar altimeter/digital terrain database if the terrain below is not perfectly horizontal. A navigation error may cause the altimeter to select the wrong location in the digital terrain database. The magnitude of this error is directly related to the slope of the terrain. In addition, nearby terrain can affect the altitude measurements. Radar altimeters radiate their pulses in a conically shaped pattern below the aircraft. The radar altimeter signal may be affected, for example, by nearby hills to the left or right of the aircraft's flight path.
Integrating the radar altimeter/digital terrain database calculated altitude with an inertial reference system (IRS) can smooth out localized terrain variations, for example, those that might be caused by construction of a new building. However, integrating the radar altimeter/digital terrain database calculated altitude with an inertial reference system cannot effectively eliminate correlated errors in a calculated altitude that might occur over a longer distance, for example, those errors that occur over several miles such as caused by seasonal variations within an expanse of forest.